Many modern communications and measuring systems are assembled from a number of smaller subsystems or stations that are geographically spaced from each other and that are arranged to work together. One such system is a paging system that typically comprises a paging terminal, a paging system controller, and a number of transmitter units, called paging stations, that are located over a wide geographic area. The paging terminal is connected to the publicly switched telephone network and receives incoming calls to the system subscribers. In response to a call, the paging terminal formulates a page for the subscriber and forwards the page to the stations through the paging system controller. The paging stations, upon receipt of the page, broadcast it over their transmitting equipment. The subscriber's pager, which is a small receiver, picks up the broadcasts and, by the actuation of a display or generation of an audio tone, notifies the subscriber that he/she has been paged. Other types of multistation systems are data acquisition systems that include a number of monitoring sites for measuring a particular parameter, such as wind or seismic motion. Moreover, telemetry systems, which are systems used to obtain data and forward it to distant locations, often are comprised of spaced-apart subsystems that are designed to act together.
For many multistation communications and measuring systems to function properly, each station must include a control clock, or timer, and all the clocks must be synchronized. In other words, each of the clocks must, at the same moment, indicate the same time. For example, one paging system is arranged so that the paging system controller collects a number of pages, bundles them together in a packet, and then forwards the packet to the paging stations along with an instruction indicating when the packet should be broadcast. The paging stations then broadcast the packet of pages at the time indicated in the instruction. As long as all the stations broadcast the packet at the exact same time, pagers carried by system subscribers who are in areas where pages from two or more stations can be received will essentially receive a single signal that the pagers' circuitry can readily process. However, if the pages are broadcast at different times, the pagers will receive multiple, overlapping signals that cannot be processed. As a result, when a subscriber carries a pager into one of these signal overlap zones, it becomes, in effect, useless. In order to avoid this undesirable result, it is desirable for all the paging stations to have clocks that indicate the same time so that each station transmits the same packet of pages at the same time.
To date, it has proved difficult to provide a set of spaced-apart locations, such as paging stations, with clocks that are all in synchronization. The individual stations can be provided with very accurate crystal-controlled clocks that are periodically synchronized to a common reference time. A disadvantage of this practice is that the high-accuracy crystal-controlled clocks are very expensive. Moreover, even if these clocks are provided, it is still necessary to provide some type of synchronization equipment at each clock site in order to ensure that all the clocks run at the same rate. Furthermore, it is typically necessary that the synchronization of these clocks be performed by a technician who visits the clock site. The expenses associated with having personnel make such visits often means that such synchronization occurs at a less than optimal frequency.
Other attempts at providing a multiclock synchronization system have involved providing a master unit that generates a continuous reference signal and a set of clock drive circuits that use the reference signal to regulate the advancement of the clock units associated therewith. Typically, the reference signal is some type of AC signal and the clock drive circuits employ phase-locked loop subcircuits to regulate the advancement of clock advance signals. A disadvantage of these systems is that it has proved difficult to continually forward a reference signal to the individual clock sites. Given the scarcity of unassigned radio frequencies, there are many locations where it is essentially impossible to establish a radio link for generating such a reference signal. In these locations it would be necessary to forward the signal by a land link, such as a conventional wire line or a fiber-optic transmission link. While such lines can readily be used to forward a reference signal, the cost of connecting them to many locations can be expensive. As the number of clock sites intended to be synchronized increases, the expense of providing such a hard wire link can grow to the point of being cost prohibitive. Moreover, many of these systems require that the individual stations receive the signals in a specific phase relationship to each other. When the signal is transmitted to the individual stations over the publicly switched telephone network, the carrier may, from time to time, modify the routing of the signal to the individual stations. The inherent change in signal propagation time to the individual stations results in the phase relationship of the signal received at the station to shift. This necessitates having to adjust the processing equipment at the station in order to ensure that the signal is processed in the appropriate phase relationship.
Still another disadvantage of many current clock synchronization systems is that they are not well suited for use at clock sites that the user wants to establish only on a temporary basis or for use with a portable clock. Owing to their sensitivity, crystal-controlled clocks must be recalibrated, their frequency reset, each time they are set up. Moreover, owing to their size and power requirements, they do not lend themselves to installation in a portable housing, such as an instrument truck. Clocks controlled by constant-reference signals have similar problems. These clocks cannot be moved unless there is some assurance that the clock drive circuits will always be able to receive the requisite reference signals. It has proved very difficult to continually provide these signals, either when the clock is moved from site to site or when the clock is actually in motion.